Functionally graded cemented carbides (FGCCs) have an excellent combination of high hardness and high toughness. But the appearance of residual stresses, resulting from cobalt gradient and mismatch between mechanical properties and material constituents, is an influence on the lifetime of FGCCs. To get the thermal stress field distribution, a constitutive equation for WC-Co FGCCs is developed by redefinition of elastic constraint factor and introduction of plastic constraint factor. The constitutive model is applied to thermal stress analysis of WC-Co composites. The distribution of thermal stresses in WC-Co specimen is obtained by means of finite element numerical method. Simultaneously, the surface compressive stress of FGCCs alloy is measured by X-ray diffraction method. Numerical results show the thermal stresses of WC-Co composites mainly concentrate in the cobalt gradient zone and the maximum value of principal compressive stress is 380MPa in the surface zone. This results is good agreement with X-ray diffraction measurement.